Plant with a pump unit for circulating heated liquid within the plant

Abstract: In a plant comprising a pump unit for circulating heated liquid within the plant, said pump unit comprising a liquid pump (1,2,3,5) driven by an electric motor (4,6,7), the operating economy is improved by cooling the electric motor by circulating a part-flow of the heated liquid circulated by the liquid pump in a cooling jacket (8) provided in the electric motor (4,6,7). A further improvement is achieved by thermally insulating (10) the electric motor (4,6,7) and the liquid pump (1,2,3,5) from the surroundings, so that the heat losses from the electrical components are utilized for heating the liquid being circulated.

Description: TECHNICAL FIELD The present invention relates to a plant with a pump unit for circulating heated liquid within the plant, said pump unit comprising a liquid pump and an electric motor driving the liquid pump. BACKGROUND ART In plants of this type, e.g. a central-heating plant, circulating pumps of the so-called wet-rotor type are conventionally used, said pumps being cooled in part by the circulating liquid in the region between the rotor and the stator, in part by the ambient air in the region around the motor housing. Alternatively and especially in connection with larger pumps, a motor with a dry-running rotor is used, the motor being air-cooled, this cooling conventionally being augmented by a blower rotor mounted on the motor shaft. Both these types of circulating pumps dissipate a substantial amount of heat to the surroundings, thus constituting a source of undesired heating of the space, in which they are situated. The dissipated heat comes in part from the motor losses, in part from the heated liquid being circulated. Normally, no provision is made for insulating such circulating pumps, as it is necessary for the motor to give off heat to the surrounding air in order to prevent it from becoming too hot. Besides, it is usually difficult and time-consuming to perform maintenance and inspection work on a thermally insulated pump unit. EP-A-0 467 282 describes a pump unit, which is cooled by circulating a part-flow of the pumped liquid in a cooling jacket. The pumped liquid is mentioned as being waste water or sludge. Accordingly there is no intentional heating of this liquid and the temperature can be expected to be in the range 5-15°C. This document does not mention anything about recovery of heat or preservation of energy. The only object of this document is to cool the motor and to remove air from the cooling jacket. US-A-4 854 373 discloses a pump unit for circulating heated liquid. The motor is partially cooled by circulating a part-flow of the circulated liquid in a separate externally mounted cooling jacket covering approximately one half of the surface of the motor. Since the flow of the liquid in the cooling jacket can be controlled, the motor has to be cooled in an alternative way, when this control reduces the cooling via the cooling jacket. This part of the cooling represents loss of energy to the surroundings. DE-A-31 09 624 describes thermally insulation of a pump unit with the purpose of transporting the heated water with minimum heat loss. This document does not mention anything about recovery of the losses in the motor, to achieve a substantial energy saving. The pump unit is a so-called wet-rotor type whereas the present invention relates to a pump unit with a dry running rotor. DISCLOSURE OF THE INVENTION It is the object of the present invention to provide a plant of the kind referred to initially, but having an improved economy of operation, as the heat losses from the electric motor are utilized for heating the heated liquid being circulated. This object is achieved according to the invention by means of the features set forth in the characterizing clause of claim 1. Since the temperature of the circulated liquid in such plants is usually within the interval 60-100° C, it is possible by suitably dimensioning the liquid flow in the cooling jacket to keep the temperature of the motor on an acceptable level. A further improvement of the operating economy is achieved by insulating the complete pump unit, this having been made possible by the fact that it no longer depends on air cooling to keep the temperature at a suitably low level. Advantageously, this insulation is provided by means of a set of insulating shells, preferably made of polyurethane foam, mounted around the pump unit, making it possible to perform maintenance and inspection work on the pump unit by simply removing the insulating shells. BRIEF DESCRIPTION OF THE DRAWING In the following detailed portion of the present description, the invention will be explained in more detail by way of example with reference to the drawing, showing in section a pump unit for use in a plant according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT The pump unit shown on the drawing comprises a pump housing 1 with an inlet connection 2 and an outlet connection 3 for the heated liquid being circulated. Inside the pump housing 1, a pump impeller 5 is mounted on a motor shaft 4. The motor driving the pump is a conventional electric motor, e.g. a three-phase asynchronous motor, with a stator 6 and a rotor 7 on the motor shaft 4. The motor is provided with a cooling jacket 8 embracing the stator 6. Cooling ducts 9 for the circulation of cooling liquid are provided in the cooling jacket 8, said cooling liquid being taken from the pressure side of the pump and returned to its suction side. The pump unit is insulated from the surroundings by means of a set of insulating shells 10, surrounding or embracing the pump unit completely with the exception of openings for the inlet and outlet connections 2,3, for electrical connections to the stator windings and for access with a view to expelling air from the cooling ducts. In a plant of the kind referred to initially, a saving in operating costs is achieved, if a pump unit as described above is made to replace the traditional circulation pump, in which the motor is cooled by air, this cooling being augmented by means of a blower rotor mounted on the motor shaft. This is in part due to the higher efficiency of the electric motor, not having to drive a blower rotor mounted on its shaft, in part that the heat losses of the motor are transferred to the circulating liquid, thus effecting a corresponding saving in oil or natural gas for heating the liquid. A further reduction of the heat losses to the surroundings is achieved by the fact that the pump unit as a whole is insulated from the surroundings. EXAMPLE A pump unit according to the prior art, comprising an un-insulated liquid pump and a blower-cooled electric motor, exhibits the following operating costs, the price of electricity being set to 1 kr./kWh and oil or natural gas, account taken for boiler efficiency, is set to a price of 0.50 kr./kWh (all amounts in Danish kr.). Efficiency of blower-cooled motor: η = 0.82. Consumption: 3 kW/0.82 = 3.65 kW. Cost per hour: 3,65 kW - 3.65 kr. Temperature of circulated liquid: T = 100° C Ambient temperature: t = 20° C Surface area of the pump housing: 0,20 m 2 Heat-transmission coefficient: α = 0.023 kW/°C m 2 Heat loss from pump housing: 0.023 x 80 x 0.20 = 0.372 kW Corresponding to fuel consumption per hour: 0.372 x 0.50 = 0,19 kr. Total operating cost per hour: 3.84 kr. Since the 3 kW of mechanical power being delivered to the pump shaft is consumed in losses in the pump and pumping work, all being converted to heat in the plant, the resulting saving in fuel is 3 x 0.50 = 1.50 kr., giving a corrected value of the operating cost per hour of 3.84 kr. - 1.50 kr. = 2.34 kr. Solely by replacing the blower-cooled motor by a jacket-cooled motor, an improved efficiency for the motor is achieved, viz. η = 0.90, i.e.: The motor losses are transferred to the circulating liquid, i.e. 3.33 kW - 3 kW = 0.33 kW, thus effecting a saving per hour of 0.33 x 0.50 = 0.16 kr. Total operating cost per hour: 3.33 + 0.35 - 0.16 = 3.52 kr. Corrected operating cost per hour: 2.02 kr. In order to reduce the heat losses to the surroundings, i.e. the above-mentioned 0.35 kr. entered as "Total cost of heat losses", the pump unit is insulated as shown by means of a set of insulating shells of polyurethane foam. The thickness of the insulating material is 60 mm and the surface area of the insulating jacket is 0.78 m 2 . The following parameters apply to polyurethane foam: Heat conductivity λ = 0.02 x 10 -3 kW/m°C Internal transfer resistivity m i = 129 m 2 °C/kW Air-gap transfer resistivity m l = 172 m 2 °C/kW External transfer resistivity m u = 43 m 2 °C/kW 1:k = (129 + 172 + 43 + 0.06/0.02 x 10 -3 ) m 2 °C/kW = 3344 m 2 °C/kW causing that heat loss: k = 0.3 x 10 -3 kW/m 2 °C 0.78 x 80 x 0.3 x 10 -3 kW = 0.019 kW Corresponding to fuel consumption per hour: 0.01 kr. Total operating cost per hour: 3.33 + 0.01 - 0.16 = 3.18 kr. Corrected operating cost per hour: 1.68 kr. Thus, a saving is achieved in the operating cost per hour of 2.34 kr. - 1.68 kr. = 0.66 kr. Assuming that the pump unit is in operation for 220 full days and nights per year, the total operating time will be 220 x 24 = 5280 hours per year, thus effecting a yearly saving of 5280 x 0.66 = 3484,80 kr. As the price of the pump is of the order of magnitude 6000 kr., at least the increase in cost for the special embodiment will have paid for itself in the course of one year. LIST OF PARTS 1 Pump housing 2 Inlet connection 3 Outlet connection 4 Motor shaft 5 (Pump) impeller 6 Stator 7 Rotor 8 Cooling jacket 9 Cooling ducts 10 Insulating shells

Claims: Plant comprising a pump unit for circulating heated liquid within the plant, said pump unit comprising a liquid pump (1,2,3,5) and an electric motor (4,6,7) driving said liquid pump, characterized in that the electric motor is exclusively cooled by circulating a part-flow of the heated liquid circulated by the liquid pump in a cooling jacket (8) provided in the electric motor, and that the electric motor (4,6,7) and the liquid pump (1,2,3,5) are thermally insulated (10) from the surroundings. Plant according to claim 1, characterized in that the thermal insulation (10) of the electric motor (4,6,7) and the liquid pump (1,2,3,5) is provided by means of a set of insulating shells (10) mounted around the pump unit. Plant according to claim 2, characterized in that the insulating shells (10) are made of polyurethane foam.

Inventors: RASMUSSEN LEIF, RASMUSSEN, LEIF
Applicants: SMEDEGAARD AS, T. SMEDEGAARD A/S
Application year: 1994
Main classification: F04D29
IPC codes: F04D29, F04D13